(125b) Effects of Nitrogen Source and Concentration On the Production of Fatty Acids in Heterotrophic Chlorella Protothecoides

The recently rising oil price has generated large interests in producing biodiesels or fatty acid methyl esters as biofuels. However, the lack of oil feedstock and high cost for raw oil production significantly limit large-scale production of biodiesels. Single cell oil (SCO) from microorganisms, especially microalgae, has received considerable attention as an alternative oil source for biodiesels production because microalgae have the potential to produce up to 50% of their biomass as lipids at rates approaching 50 g of dry biomass/m2/day and oil yields per unit area more than 100 times of that from any plant system. However, photoautotrophic growth of microalgae is difficult to scale up in an industrial setting. Alternatively, heterotrophic growth of microalgae in darkness eliminates the requirement for light and facilitates large-scale production of microalgal biomass in high-cell-density cultures, and has been successfully used for efficient and low-cost production of biomass and lipid products in conventional industrial fermentation systems.

Green microalgae Chlorella species have been suggested as a potential source of biofuels with high calorific value linked to the high lipid content. The goal of this work was to develop an efficient heterotrophic fermentation process for culturing the unicellular green microalgae Chlorella protothecoides for microalgal oil production. Chlorella protothecoides has a high specific growth rate, high biomass concentration and high cellular lipid content, and is thus a good candidate for biodiesel production. In general, nitrogen limitation could stimulate the lipid accumulation in microalgae, but the yield and composition of total fatty acid are greatly affected by the nitrogen source and concentration, which need to be optimized. To optimize the fermentation, four nitrogen sources (urea, Tryptone, Bacto peptone and yeast extract) at various concentrations (0-0.1 mol N/l) were evaluated for their effects on biomass and total fatty acid (TFA) production in batch cultures. With 40 g/l of glucose as the carbon source, the highest TFA yield of 5.71 g/l was obtained with urea as the nitrogen source at 0.07 mol urea-N/l, which produced a high concentration of biomass (28.4 g/l) with a high TFA content (20.12% DW). A lower urea concentration (0.05 mol N/l) produced more biomass (30.49 g/l) but with a lower TFA content (15.70% DW). In contrast, Bacto peptone at 0.03 mol N/l gave the highest TFA content (41.71% DW) but a very low biomass concentration (3.87 g/l), resulting in a low TFA yield of 1.61 g/l. The fatty acid composition in heterotrophic cells was relatively simple, over 99% of TFA being C16 (~26.5%) and C18 (~72.5%) fatty acids. These results indicated that batch culture of heterotrophic C. protothecoides can be used to produce fatty acids suitable for biodiesel production from glucose as the carbon source and urea as the nitrogen source.